I. Field of Invention
The instant invention relates to a control system for automatically adjusting the displacement of a plurality of variable displacement pumps which are driven by a prime mover.
II. Description of the Prior Art
It is common for a prime mover to drive a plurality of variable displacement pumps which are capable of demanding and using more horsepower than the prime mover can provide. As a minimum requirement, it is necessary to provide a control system which will act to reduce the displacement of some or all of the pumps when the prime mover approaches an overloaded condition in order to prevent it from stalling. Such control systems are well known.
In one control system, shown in U.S. Pat. No. 3,723,026 to Soyland, the collective pump working pressures exert a force on a valve. If the force exceeds a predetermined value the valve passes pressure fluid to the pilot stages of valves which operate to disconnect one or more constant volume pumps or to control pistons which operate to reduce the displacement of one or more variable displacement pumps.
In another control system, shown in U.S. Pat. No. 3,841,795 to Ferre et al, a fixed displacement pump driven by a prime mover, which also drives a plurality of variable displacement pumps, supplies fluid to an underspeed control valve. When the prime mover begins to slow down from overloading, the underspeed valve shifts to pass pressure fluid from the fixed displacement pump to servo-control valves which set the displacement of variable displacement pumps. These valves reduce the displacement of the pumps until the overload is eliminated.
A third control system, shown in U.S. Pat. No. 3,649,134 to Wagenseil, discloses a plurality of variable displacement pumps driven by a prime mover which drives a centrifugal governor. When the speed of the prime mover decreases due to overloading, the governor operates a valve which passes working pressure fluid from one of the pumps to spring biased pistons which reduces the displacement of the pumps.
Although the prior art control systems act to prevent the prime mover from stalling they do not change the displacement of all the variable displacement pumps proportionally. It is desirable to have a system which changes the displacement of all variable displacement pumps proportionally (i.e. the displacements of all of the pumps are reduced in proportion to the setting of the individual manual displacement controls) so that the speeds of all of the consumers are diminished proportionally when the prime mover is overloaded. Such a system is essential, for example, when two pumps driven by a prime mover drive fluid motors on opposite tracks of a tracklaying vehicle. If one track is in mud and the other is on hard ground, the pump supplying the motor of the former must displace more fluid thereto in order to keep the vehicle moving in a straight line. If the prime mover becomes overloaded, the displacements of the two pumps have to be reduced proportionally (by the same percentage) in order that the proportions of their fluid flows will remain constant and the vehicle will continue in a straight line.
In Soyland and Wagenseil, mentioned above, a control valve passes pressure fluid to spring biased control pistons when the prime mover is overloaded. The pistons are connected to the displacement varying mechanisms of the pumps and operate against the springs to reduce the displacement of the pumps. With this arrangement, if the pump displacements are different, they cannot be reduced uniformly by the same percentage. This is because when the pumps are at different displacements the spring forces on the control pistons are different. Consequently, when pressure fluid acts on the pistons, the one with the least spring force exerted thereon will move first and the displacement of its pump will be reduced disproportionally with respect to the other pumps.